Significant effect of chloride ions on the electrogenerated chemiluminescence (ECL) behavior of the ruthenium(II)tris(2,2'-bipyridine) (Ru(bPY)(3)(2+))/tri-n-propylamine (TPrA) system at a Au electrode was reported. At low concentrations (e.g., vertical bar Cl-vertical bar < 5 mM), the ECL was enhanced; at relatively high concentrations, however, the ECL intensity decreased with the increase of the vertical bar Cl-1 vertical bar. At vertical bar Cl-vertical bar = 90mM, similar to 50% and 100% ECL inhibition was observed for the first and the second ECL wave, respectively. The electrogenerated gold-chloride complexes (AuCl2- and AuCl4-) Which were verified using an electrochemical quartz-crystal microbalance (EQCM) method were found to be responsible for the ECL inhibition. This study suggests that care must be taken when a Au working electrode is used for ECL studies in chloride-containing buffer solutions (widely used in DNA probes) and/or with the commonly used chloride-containing reference electrodes since in these cases the ECL behavior may significantly disagree with that obtained using other electrodes and reaction media. (c) 2008 Elsevier Ltd. All rights reserved.